Operators of mobile systems, such as Universal Mobile Telecommunications Systems (UMTS) and its offspring including Long Term Evolution (LTE) and LTE-Advanced, continue to rely on advanced features for improving the performance of their radio access networks (RANs). For improving the performance of uplink transmissions (i.e., transmissions from the mobile station or user equipment (UE), to the base station or evolved Node B (eNB)), one such feature is uplink power control. Uplink power control facilitates adjusting the transmit power of the UE, to ensure that the power level is set sufficiently high to meet the desired transmission characteristics (i.e., the desired modulation, coding rate, etc.), but is not excessively high to cause unnecessary interference to transmissions from other UEs in the network.
RANs may employ uplink power control in an open loop manner, i.e., without any explicit feedback from the eNB to the UE with regards to the power level at which the UE should transmit on the uplink. While such systems have the benefit of simplicity, open loop power control (OLPC) is generally a sub optimal approach. In particular, it often results in overprovisioning of the UE transmit power, causing excessive system-level interference and lowering the UE battery life. With only OLPC in place, such overprovisioning is, however, often necessary, as high transmit power is required to guarantee achievability of the highest SNR needed for maximizing link throughput, for countering any uplink-downlink path loss imbalance, and for overcoming uplink interference.
Closed loop power control (CLPC) has also been attempted, and provides for power control feedback from, e.g., an eNB to the UE. A feedback command (also referred to as a transmit power control (TPC) command) instructs the UE to apply an appropriate adjustment to its transmit power level. In practical systems, such as 3GPP LTE, the TPC command belongs to one out of a discrete set of possible values.
In particular, one procedure that may be employed for implementing CLPC in practice is depicted in the flowchart of FIG. 1, and is based on the algorithm proposed in “A Simple Distributed Autonomous Power Control Algorithm And Its Convergence”, by Foschini and Miljanic, IEEE Transactions On Vehicular Technology, Vol. 42, No. 4 (November 1993). The procedure is followed independently for each eNB-UE link in the network, so this approach may also be referred to as a “distributed” power control. The procedure entails defining a desired target value for the signal to interference plus noise ratio (SINR) for the eNB-UE link, and such is referred to as dT_SINR, i.e., a desired target SINR. The desired target SINR may be, for example, a SINR required to meet a certain BLER (block error rate) criterion, for a desired choice of the transmission modulation and coding (MCS) parameters. Based on the current received signal from the UE, e.g., such as uplink data symbols, uplink reference symbols, etc., the eNB measures a current achieved SINR, termed a_SINR(t) (step 12), with t denoting the current time instant. Depending on the difference in the target SINR and the current achieved SINR value, i.e., depending on (dT_SINR−a_SINR(t)), the eNB feeds back a TPC command (denoted TPC(t)) to the UE (step 14).
In a practical set-up, where the set of possible TPC values is fixed a priori (as in 3GPP LTE), the eNB may map the difference (dT_SINR−a_SINR) to the nearest entry in the set of TPC values, and feed it back to the UE. The UE then applies the received TPC command to modify the transmit power on subsequent transmissions until instructed otherwise (step 16).
The power control approach of FIG. 1 may be applied independently (and in parallel) for each eNB-UE link. At any point of time, the network may contain a set of these links, with each link attempting to achieve its own desired target SINR.
Generally, using the approach of FIG. 1, each link achieves its target SINR, provided that the set of target SINRs is jointly feasible to begin with. In other words, if the channels between the different eNBs and the different UEs are such that it is possible to achieve the desired target SINRs for all the links using a certain set of UE transmit power levels, the approach in FIG. 1 would likely lead the UEs to transmit at these required power levels.
This Background is provided to introduce a brief context for the Summary and Detailed Description that follow. This Background is not intended to be an aid in determining the scope of the claimed subject matter nor be viewed as limiting the claimed subject matter to implementations that solve any or all of the disadvantages or problems presented above.